1. Field of the Invention
The present invention relates, generally, to a transmission shifting control strategy and, more specifically, to a control strategy for load sharing between a friction clutch and one-way clutch to effect low and reverse gear ratios in a transmission.
2. Description of the Related Art
Generally speaking, land vehicles require three basic components. These components include a power plant (such as an internal combustion engine), a power train and wheels. The power train""s main component is typically referred to as the xe2x80x9ctransmission.xe2x80x9d Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. Transmissions include one or more gear sets which may include an inner gear, intermediate planet or pinion gears which are supported by their carriers, and outer ring gears. Various components of the gear sets are held or powered to change the gear ratios in the transmission. In addition to such planetary gear sets, driveline components may further include multi-disc friction devices that are employed as clutches or brakes. The multi-disc pack clutch is a friction device that is commonly employed as a holding mechanism in a transmission.
The multi-disc pack clutch or brake assembly has a clutch sub-assembly including a set of plates and a set of friction discs that are interleaved between one another. The plates and friction discs are bathed in a continual flow of lubricant and in xe2x80x9copen packxe2x80x9d operation normally turn past one another without contact. The clutch or brake assembly also typically includes a piston. When a component of a gear set is to be held, as for example during a particular gear range, a piston is actuated so as to cause the plates and friction discs to come in contact with respect to one another. In certain applications, it is known to employ several multi-disc pack clutch devices in combination to establish different drive connections throughout the transmission to provide various gear ratios in operation, or to brake a component.
When the discs are not engaged, there often remains a differential rotational speed of the drive and driven members which the clutch or brake bridges. Relative rotation between the friction discs and the plates during open-pack mode creates drag. This condition results in parasitic energy losses, reduces the efficiency of the transmission, and ultimately results in lower fuel efficiency.
In addition to multiple friction devices, one-way clutches are frequently employed in transmissions to selectively transmit torque in one rotational direction, but not in the opposite rotational direction. To this end, one-way clutches typically include an inner race, an outer race, and an engagement mechanism disposed therebetween. The engagement mechanism is operable to lock the inner and outer races together thereby transmitting torque in one relative direction. The engagement mechanism is further operable to allow freewheeling rotation between the inner and outer races in the opposite rotational direction. Engagement mechanisms commonly used in one-way clutches of the related art include pawls, sprags, and rollers. A cage, along with biasing members, such as springs, are also sometimes employed to retain the pawls, sprags, or rollers between the inner and outer races as well as to selectively assist in the change of operational modes between torque translation and freewheeling actuation of the clutch, depending on the direction of rotation between the inner and outer races.
As noted above, one-way clutches of this type have been employed in numerous applications in transmission, transfer cases, and differentials. For example, one-way clutches have been employed in conjunction with multiple friction clutches and planetary gear sets to effect low and reverse gear ratios in conventional transmissions. While this arrangement has worked well for its intended purpose, some disadvantages remain. For example, the friction clutch remains a source of significant parasitic losses due to inherent drag between the friction plates when the clutch is operating in xe2x80x9copen packxe2x80x9d mode. Still, the clutch is necessary for providing the proper holding torque in low and reverse gears. Accordingly, there remains a need in the art for a shift control strategy that activates the friction clutch to provide the appropriate holding torque for both low and reverse gears in the transmission and yet yields lower manufacturing costs and space requirements than those presently attributable to the one way clutch and multiple plate friction clutch currently used for this purpose. In addition, there is a need in the art for a control strategy that provides for a non-synchronous up-shifting from first to second gear may be effected.
The disadvantages of the related art are overcome in a transmission shifting control strategy for load sharing between a low friction clutch and a one-way clutch to provide low and reverse gears in a transmission. The control strategy includes a method of controlling an automotive transmission having at least one shaft and at least one gear set operatively coupled to the shaft and adapted to provide low and reverse gear ratios. The gear set includes a sun gear operatively coupled to a source of torque in the transmission assembly, a ring gear mounted for rotation about the sun gear and a plurality of pinion gears supported by a carrier for meshing rotation about the sun gear and between the ring gear and the sun gear. The carrier is operatively coupled to the shaft. In addition, the transmission has a friction clutch assembly including a clutch pack that acts as a holding device and as well as a one way clutch assembly that is interposed between the friction clutch assembly and the gear set. The method includes the steps of selecting a low gear ratio that is provided by the gear set. The friction clutch is actuated to ground the outer race of the one-way clutch assembly and thus the ring gear to the transmission housing. Torque is provided to the sun gear to drive the pinion gears in meshing relationship about the sun gear to transfer torque at a reduced ratio to the carrier and therefore the shaft. Activation of the friction clutch is maintained until the peak torque transmitted through the gear set has been reached. In addition, the method of the present invention includes reducing the load capacity of the friction clutch assembly while the transmission assembly is still in the low gear ratio defined by the gear set and so that the one way clutch assembly acts as the sole holding device on the ring gear of the gear set such that a non-synchronous shift from the low gear to the high gear may be effected.
In this way, the control strategy may be employed to provide load sharing between a friction clutch and a one-way clutch to provide low and reverse gear ratios. The control strategy may result in a reduced capacity one-way clutch that yields lower manufacturing costs and has reduced space requirements when compared to one-way clutches known in the related art. When used in this way, the control strategy results in smooth non-synchronous up-shift from first to second gears.
Other objects, features and advantages of the present invention will be readily appreciated as the same becomes better understood after reading the subsequent description taken in connection with the accompanying drawings.